Compressor

ABSTRACT

A compressor for pressurizing a fluid characterized by opposed reciprocating pistons or cylinders driven by linear electric motors. Movable pistons or cylinders are made to operate along a common axis 180* apart to provide high pressure fluid to the refrigerator. In the operating mode, the pistons move simultaneously in opposite directions. The apparatus includes in one embodiment means for reciprocating the cylinder relative to a fixed piston, the compressor in either mode having controls to limit the stroke thereby minimizing contact of the piston with the end of the cylinder.

United States Patent 1 1111 3,910,729

Jepsen et al. Oct. 7, 1975 [5 COMPRESSOR 3,515,966 6/1970 Valrogeretal ..417 418 1751 ROME-Jew, Emmausueanc- 3332312331 31333 fiflglifiiiiiffliiji.......::1:11:1131i Liberty, Allentown; Richard E.

Luybh Hellertown of Primary ExaminerC. J. Husar [73] Assignee: Air Products and Chemicals, Inc., Attorney, Agent, or FirmJames C. Simmons; Barry Allentown, Pa. Moyerman [22] Filed: June 25, 1973 [21] Appl. No.: 373,084 [57] ABSTRACT A compressor for pressurizing a fluid characterized by opposed reciprocating pistons or cylinders driven by [52] 417/417 f g i fl g linear electric motors. Movable pistons or cylinders 51 l t Cl /66 are made to operate along a common axis 180 apart 17/04 to provide high pressure fluid to the refrigerator. In 1 0 care 17/ 4 41 the operating mode, the pistons move simultaneously 17/41 460 in opposite directions. The apparatus includes in one embodiment means for reciprocating the cylinder rela- [56] References cued tive to a fixed piston, the compressor in either mode UNITED STATES PATENTS having controls to limit the stroke thereby minimizing 2,988,264 6/196! Reutter 417/418 contact of the piston with the end of the cylinder.

3,162,134 12 1964 Lovell 417/418 3,303,990 2/1967 Curwen 417/418 20 clalms, 6 Drawmg Flgllres IIII\IIIIIIIIIIIIIII x -lllll WWII Sheet 1 of 3 3,910,729

US, Patfint Oct. 7,1975

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COMPRESSOR BACKGROUND OF THE INVENTION This invention pertains to compressors for raising the internal pressure of a fluid to a high level and, in particular, for pressurizing fluids to be used in refrigeration systems. A refrigeration compressor is used to raise the pressure of a working fluid such as a refrigerant gas, which gas is then expanded through a nozzle to a lower pressure, thereby absorbing heat and producing refrigeration by the expansion. In normal closed cycle refrigeration systems, this gas, after expansion, is recompressed and recycled through the expansion orifice to further produce refrigeration. The most common type of refrigeration compressor is the reciprocating type having one or more pistons attached to a crankshaft for movement in a corresponding cylinder or cylinders for compressing the gas therein. Such rotary compressors contain a crankshaft and flywheel with the attendant bearings, connecting rods with wrist pins, gears, and the like. Such compressors operate usually at high noise levels and are subject to severe vibration loading and for ultra-high pressure applications, such compressors tend to be extremely large.

US. Pat. Nos. 2,679,732, 2,839,273, and 2,061,869 are examples of a type of compressor wherein the pistons are driven along the linear axis of the compressor housing by an electric motor. These compressors operate with both pistons moving in the same direction simultaneously to produce a supply of pressurized working fluid to the refrigeration source. Such compressors tend to create vibration-type forces that require special mounting structures and are generally elongated requiring considerable external piping with a relatively long stroke and must be operated by an alternating current source.

SUMMARY OF THE INVENTION In order to avoid the above-described problems both with the reciprocating type and the heretofore disclosed linear-type compressors, it has been discovered that a linear compressor can be achieved wherein two opposed pistons operate 180 apart along a common axis with each piston driven by a separate linear motor.

The pistons are integral with the motor coil and guide shaft and are fabricated to slide in a cylinder which is integral with the motor shell and permanent magnet. The entire motor case is pressurized with a gas and the pressure is maintained at an intermediate level between the inlet and outlet pressure of the compressor to minimize the motor force necessary to operate the compressor. In addition, limit springs are located at both ends of the piston assembly to prevent contact of the top of the piston with the cylinder at the ends of the stroke. The springs can be replaced by gas cushions or a resonant spring system to further provide effective operation of the compressor. The energy expended in compressing the limit spring and the gas in the motor case outside of the cylinder is recovered following the compression stroke and the fact that the piston is driven in both directions reduces the size, weight, and power requirements of the overall compressor.

A linear compressor of this type having no flywheel, crankshaft, rotary bearings, gears, and the like, removes the primary sources of mechanical wear, failure, noise, friction, and vibration. By operating the pistons 180 apart along a common axis, the vibration and noise of the overall compressor can be essentially eliminated. The linear compressor requires bearings only to maintain alignment of the piston and cylinder and coil and magnet. Since there are no connecting rods, the number of bearings is reduced and the side loading on the bearings present is lowered thus reducing bearing wear significantly. The compressor further includes dry film lubrication of the bearings eliminating oil contamination in the compressor.

Combining the piston and motor coil and eliminating gear and crank mechanisms, provides for a small, compact and simple compressor with a minimum of precision parts, thereby resulting in simplicity, higher reliability, easy maintenance, and low cost.

Therefore, it is the primary object of this invention to provide an improved compressor.

vIt is another object of this invention to provide a linear compressor having small size and low power requirements.

It is yet another object of this invention to provide a readily fabricated, low-cost compressor with few moving parts.

It is still yet another object of this invention to provide a compressor with few bearing surfaces with significantly reduced loading on the bearings.

It is still another object of this invention to provide a compressor with operating opposed pistons running apart along a common axis to minimize vibration and noise as the compressor is operating.

It is a further object of this invention to provide a dry film lubricated compressor.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a longitudinal section taken along the line 11 of FIG. 2 of a compressor according to the present invention.

FIG. 2 is a transverse section taken along the line 2-2 of FIG. 1.

FIG. 3 is an end view of an alternate embodiment of a compressor according to the present invention.

FIG. 4 is a view taken along line 44 of FIG. 3.

FIG. 5 is a view taken along line 55 of FIG. 4.

FIG. 6 is an elevational view partially in section of a compressor according to the present invention wherein there is employed an alternate piston return mechanism.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to FIGS. 1 and 2, there is shown a compressor 10 including a cylindrical housing having a first half 12 and a second half 14. Housing section 12 includes a receiver on end 16 for receiving an instrument probe 18, whereas housing section 14 has a receiver 20 for receiving a hose or other fluid fitting 22. Disposed within the housing sections 12 and 14 and affixed thereto is a piston 24 having a longitudinal bore 26, a first piston end 28 and a second piston end 30. Surrounding piston end 28 is a first cylinder 32 and surrounding second piston end 30 is a second cylinder 34 defining a first chamber 36 and a second chamber 38 between piston and cylinder 2832 and 30-34 respectively. The cylinders 3234 are affixed to a crown member 40, 42 respectively as by a plurality of fasteners shown as 44. Affixed to the crowns 40 and 42 is a thin metal cylinder 46, 48 having a wall thickness of approximately 0.010 inches. Surrounding the shells 46 and 48 respectively are coils 50 and 52 such as normally found in a linear assembly motor as will hereinafter be more fully explained. The assembly of the cylinder 32, crown 40, thin cylindrical shell 46, and coil 50, and of the cylinder 34, crown 42, thin cylindrical shell 48 and coil 52 are each mounted for movement along the longitudinal axis of the compressor as is shown, thereby forming a variable volume between the respective piston and cylinder for chambers 36 and 38 respectively. Disposed within chambers 36 and 38 respectively at the top of cylinders 32 and 34 are rubber cushioning pads 54, 56 cushioning the contact of the re spective cylinders with the corresponding end of the fixed piston 24. At the top of cylinders 32, 34 respectively between the cylinder and the respective housing 12 and 14, there are included limit springs 58, 60 for absorbing the energy from the excursion of the respective cylinders.

Disposed within housing sections 12 and 14 and affixed thereto is a pair of magnet assemblies consisting of pole pieces 62, 64, magnets 66, 68, and washers 70, 72 respectively.

The magnet and the coil, as disclosed in US. Pat. No. 3,469,163 which patent specification is incorporated herein by reference, drive the pistons 32, 34 to provide the variable volumes 36, 38 to effect compression of a working fluid as will hereinafter be more fully described. Two guide rods and sleeve assemblies are included in the compressor to assure alignment of the moving cylinders; however, only one is illustrated at 74, and, in fact, only one is required.

Electrical feed through 76 is disposed between housing sections 12 and 14 for feeding the current through conductors 78 and 80 to the respective coils 50 and 52. The necessary electronics are adequately disclosed in the aforementioned US. Pat. No. 3,469,163.

In FIG. 2, there is shown a gas inlet conduit 82, which also functions as an outlet conduit and communicates with the longitudinal bore 26 in piston 24. In FIG. 2 is also shown a pressure transducer assembly 84 and water cooling conduits 86 (inlet) and 88 (outlet).

Within housing sections 12 and 14, there are a plurality of bumper pads 90 for cushioning the contact of the movable cylinder assembly to the fixed piston 24.

The compressor of FIGS. 1 and 2 and FIGS. 3, 4 and is ideally suited for operation to control a refrigerator as shown in US. Pat. No. 3,620,029. The inlet-outlet conduit 82 being coupled directly to the conduit 30 just above conduit 48 of the refrigerator as shown in FIG. 1 of the aforementioned patent.

The operation will be described in relation to such a refrigerator.

The transducer assembly 18 is of the linear displacement type sold by Schaevitz Engineering Co. and as Model No. AC LUDT type 500 MI-IR. Such a transducer monitors the position of the cylinder in regard to the time during a respective stroke. Pressure transducer assembly 84 (sold by Statham Instruments Inc. as type PA 208TC-lM-350) is included to determine pressure in relation to time of the cycle for monitoring the overall perfonnance of the compressor and for establishing a P-V diagram for the system.

The electronics are so wired that the respective coils 50 and 52 are wired in parallel and are set up so that the respective cylinders 32, 34 are operated 180 apart along a common axis with the pistons moving simultaneously in opposite directions, therebyminimizing the vibration and the operating noise of the compressor. The coils are energized simultaneously so that their motions are identical but opposed.

After the compressor is assembled and tested to assure there are no leaks, it can be charged through fitting 22 with the working fluid, e.g. helium, which fills the entire internal volume of the compressor. The entire internal portion of the compressor is charged to a pressure level intermediate, the minimum and maximum working pressures of the compressor. After the compressor is charged with the initial charge of helium, the fitting 22 is sealed and the compressor is ready for use.

Of course, as with all piston-cylinder combinations, a plurality of piston rings shown generally at 92 are included to prevent excessive leakage between the piston and the cylinder. A preferred type of piston ring construction is shown in US. Pat. No. 3,540,746.

In one application, the inlet-outlet conduit 82 is connected to a refrigerator of the displacer-expander type such as shown in US Pat. No. 3,620,029 and described above. The inlet-outlet fitting 82 is connected directly to the cold end of such a refrigeration apparatus and the entire system is charged with a working fluid such as helium by means of fitting 22. After charging, the system is purged of ambient atmosphere. The compressor is set in motion by energizing the coil 50, 52, causing the respective cylinders 32, 34 to move toward the ends of the housing 12, 14 respectively causing the fluid to enter into chambers 36, 38. As is shown in the drawing when the respective cylinders reach the outermost limit of their excursion, the coil is at its maximum insertion into the magnetic field. As the maximum excursion point is reached, the abrupt change in the back electromotive force (BEMF) is sensed, thus cutting off the current flow to the coil. The cylinders are then returned inwardly (toward the center of the compressor), thereby causing the fluid to be compressed in their respective cylinder volumes 36, 38. Cylinder return is accomplished by the differential pressure existing between cylinder volumes 36, 38 and the compressor housing and the stored energy in springs 58, 60. During this compression stroke, the gas exits through conduit 82. The rubber bumper pads 54, 56, and prevent harsh contact of the cylinder to the piston and housing as the cylinders reach their maximum excursion points.

In addition, the fact that the motor casing is pressurized to an intermediate level between the suction and compression pressures of the compressor results in a gas cushioning of the cylinder during both points of maximum excursion. Because of the pressure differential and the springs acting in co-operation, the electric motor does not have to exert such a large force in compressing the fluid by recovering energy expended in compressing the gas, which energy can be recovered on the expansion stroke. The only input energy required in order to maintain the cycle is that necessary to overcome the electrical resistance and hysteresis, gas throttling, irreversibility, friction, and work removed by the refrigeration device.

As shown in FIGS. 3 and 4, the compressor 10' consists of movable cylinders 32' and 34' and in basic construction, is similar to the compressor of FIGS. 1 and 2. There is shown in the compressor of FIG. 4 a guide cylinder I00 and 102 for guiding the excursion of cylin ders'32 and 34 and assuring their axial alignment. The respective cylinders 32', 34 move axially inside the cylindrical bore of the respective guide cylinders and are positioned with the aid of guide rings 104, 106 preferably fabricated from a carbon-graphite filled teflon material such as used in seals and piston rings. The guide rings 104, 106 are of sufficient axial length to prevent misalignment or wobbling of the respective cylinders during their excursion. Each guide cylinder includes a series of vent holes 108, 110 that communicate with the interior of the housing of the compressor and furthermore which communicate with the interior volume of compressor housing through vent ports 112, 114 in the respective cylinders. The vent holes-vent port fluid circulating system serves as means for cooling the compressor by transferring the heat from the helium to the outside shell of the compressor where it can be readily dissipated by air cooling or water cooling the compressor shell. This further aids in the overall effectiveness of the compressor. The return springs in the embodiment of FIGS. 3 and 4 shown respectively as 58' and 60' and are of ashorter length fitting in a suitable recess in the respective guide cylinder 100, 102 circumference thereby shortening the overall length of the compressor.

There is shown in FIG. 6 another embodiment 115 of the compressor according to the present invention. In this embodiment, the piston 117 moves relative to the cylinder 116. The piston includes a guide ring 118 and suitable piston rings 120 such as shown in US Pat. No. 3,540,746. On the head of the piston 117 is an inlet valve 122 for admitting gas into the piston chamber from the center of the piston stem 124 as is shown in the embodiments of FIGS. l-5. In the head of the cylinder are suitable discharge valves 126 for providing the pressurized fluid to a point of use. In this aspect, the compressor of FIG. 6 operates in the traditional manner of fluid compressor. There is shown disposed on either side of magnet pole piece 62' springs 130 and 132. The springs 130 and 132 are selected to have the same spring constant in that they exert equal force; the spring 130 being the compression stroke spring and the spring 132 being the suction stroke spring. The springs are sized so that at the half-way point each spring is exerting an equal force and the piston would tend to remain at rest. In this way, the springs act as a resonant spring mass system and the coil or motor drive need only overcome the friction forces of the guide ring and the piston rings together with any windage forces that are available and the force to compress the gas, thereby resulting in a more compact compressor. The compressor of FIG. 6 is identical to the compressors of FIGS. l-4 in that it has a piston and cylinder on either end which operate 180 apart along a common axis. The spring mass system approaches a resonant spring system, thereby enabling the fabrication of a dry lubricated compressor; only the guide ring and the piston rings being subject to wear. Such compressors need no oil lubrication and can have a short stroke dry lubricated motor coupled to a short stroke dry lubricated compressor.

The dual spring system illustrated in relation to FIG. 6 can be used with the devices of FIGS 1 through 4. It is also possible, and in fact, may be advantageous to have springs of unequal spring constant with a'c ompressor such as shown in FIGS. 1 through 4 because of the differential fluid pressure between the cylinder volumes and the compressor housing.

From the foregoing description, it would beapparent to a worker skilled in the art that the arrangement of springs and coil force could be such as to have the coil driving in both directions with or without springs. Also, it may be advantageous in certain applications to have one spring assisting the coil force or employed to return the moving mass. Two springs, either of equal or unequal spring constant, can be used to work with the coil or to return the coil during the unenergized portion of the cycle.

In operation, the moving pistons or cylinders can be arranged so that at points of maximum excursion they contact the elastomeric pads. It is also possible to operate the compressor so that such contact is limited to either the maximum compression or suction stroke, or contact is eliminated altogether.

It is also within the scope of the present invention to provide for sensing of the position of the movable member (piston or cylinder) and incorporating such sensor in the electrical circuitry to control energizing of the coil. Such sensors include ultrasonic devices, proximity probes, accelerometers, limit switches and the like.

In constructing compressors according to the foregoing invention, it has been shown that vibration and noise are balanced by using the two pistons operating apart along a common axis and moving simultaneously in opposite directions. With no rotary motion, there is no need for rotary bearings, gearing, connecting rods and associated wrist pins, flywheels, thereby eliminating any forces caused by such mechanical devices. The absence of such mechanical devices eliminates the primary sources of mechanical wear, failure, noise, friction, and vibration in conventional reciprocating or rotary-type compressors. Such compressors can be constructed with a minimum of precision parts resulting in low production costs. a

It is within the scope of the inventive concept to provide a compressor such as shown in FIG. 6 wherein the operating pressures at either end are different. For ex ample, the working pressures for one piston cylinder combination could be 15 pounds per square inch (psi) suction and 60 psi compression, and the other piston cylinder combination 60 psi suction and 240 psi compression. In this instance, the piston return could be accomplished with an internal housing pressure less than the suction pressure of either one of the piston cylinder combination or greater than the compression pressure of either piston cylinder combination.

Having thus described our invention, what is desired to be secured by Letters Patent of the United States is set forth in the following claims.

We claim:

1. A compressor for pressurizing a fluid comprising in combination:

an elongate generally cylindrical closedhousing having a major and minor axis "and a first and second end defining first and second chambers in said housing;

a first piston and cylinder disposed in the first chamber and mounted for relative reciprocation thereby defining a first variable volume fluid chamber;

a second piston and cylinder disposed in the second chamber and mounted for relative reciprocation thereby defining a second variable volume fluid chamber;

the first and second piston and cylinder combination v mounted relative to each other for reciprocation along the major axis of the housing;

means associated with each. independent piston and cylinder combination to causereciprocating motion of each piston and cylinder independent of the other; and

means fior admitting and removing a fluid to the first and second variable volume chambers.

2. A compressor according to claim 1 wherein there is included means for limiting the excursion between each piston and cylinder.

3. A compressor according to claim 1 wherein there is included means for cushioning the impact of the movable piston or cylinder as they reach the points of maximum excursion.

4. A compressor according to claim 1 wherein the first and second pistons are fixed and the first and second cylinders are reciprocated in relation thereto.

5. A compressor according to claim 1 wherein the first and second cylinders are fixed to the housing and the first and second pistons are reciprocated in relation thereto.

6. A compressor according to claim 1 wherein the means for causing reciprocating motion between the piston and cylinder includes a linear motor having a coil mounted for" movement into and out of a magnetic field.

7. A compressor according to claim 3 wherein the impact cushioning means includes means for pressurizing the compressor housing with a working fluid at a pressure intermediate that of the maximum and minimum pressure in the variable volume during operation.

8. A compressor according to claim wherein the impact cushioning means further includes limit springs and elastomeric bumperpads.

9.-A compressor according to claim 6 wherein there is included 'means for limiting the excursion between each piston and cylinder said means including at'least twopi'el oaded springs acting as a resonant spring-mass system and the coil is energized only during the portion of the cycle-when the coil is entering the magnetic field.

1'0. Aco mpressor according to claim 6 wherein the coilis mounted for movement so that it can be select'ively energized during the portion of the stroke when the variable volume is decreasing, during the portion of the'stroke when the-variable volume is increasing, or

. during the entire reciprocation cycle.

1 1'. A compressor forpressurizing a fluid comprising in combination:

an elongate generally cylindrical closed housing having a major and minor axis and first and second ends defining first and second chambers in said housing;

i a first piston and cylinder disposed in the first cham ber and mounted for reciprocation of the piston and the cylinder-relative to each other thereby defining a first variable volume fluid chamber;

a second piston and cylinder disposed in the second chamber and mounted for reciprocation of the piston and the cylinder relative to'each other thereby defining a second variable volume fluid chamber;

the first and second piston and cylinder combination mounted relative to each other for reciprocation along the major axis of the housing;

means associated with each independent piston and cylinder combination to cause reciprocating motion of each piston and cylinder independent of the other;

means for admitting a fluid to the first and second variable volume chambers;

means for removing compressed fluid from the first and second variable volume chambers;

means for controlling the emission and removal of fluid from each of the first and second variable volume chambers; and

means for cushioning impact of the movable piston and cylinder as they reach the points of maximum excursion, said means includes pressurizing the compressor housing with a working fluid at a pressure intermediate that of the maximum and minimum pressure in the variable volume chambers during operation of the compressor.

12. A compressor according to claim 1 1 wherein the impact cushioning means further includes limit springs and elastomeric bumper pads mounted for cooperation with the gas cushioning.

13. A compressor according to claim 1 1 wherein the means for causing the reciprocating motion between the piston and cylinder includes a linear motor having a coil mounted for movement into and out of a magnetic field.

14. A compressor according to claim 13 wherein there is included means for limiting the excursion between each piston and the cylinder said means including at least two preloaded springs acting as a resonant spring mass system and the coil is energized only during the portion of the cycle when the coil is entering the magnetic field. f

15. A compressor according to claim 13 wherein the coil is mounted for movement so that it can be selectively energized during the portion of the stroke when the variable volume is decreasing, during the portion of the stroke when the variable volume is increasing or during the entire reciprocation cycle.

16. A compressor for pressurizing a fluid comprising in combination:

an elongate generally cylindrical closed housing having a major and minor axis in a first and second end defining first and second chambers in said housing;

a first piston and cylinder disposed in the first chamber and mounted for relative reciprocation thereby defining a first variable volume fluid chamber;

a second piston and cylinder disposed in the second chamber and mounted for relative reciprocation thereby defining a second variable volume fluid chamber;

the first and second piston and cylinder combination mounted relative to each other for reciprocation along the major axis of the housing;

means associated with each independent piston and cylinder combination to cause reciprocating motion of each piston and cylinder independent of the other, said means including a linear motor having a coil mounted for movement into and out of a fixed magnetic field;

means for admitting a fluid to the first and second variable volume chambers;

means for removing compressed fluid from the first and second variable volume chambers;

means for controlling the admission and removal of fluid from each of the first and second variable volume chambers; and

means for limiting the excursion between the piston 19. A compressor according to claim 16 wherein the and cylinderv coil is mounted for movement so that it can be selec- A compressor according to Claim 16 wherein Said tively energized during the portion of the stroke where means limiting the excursion between each pis ton the variable volume is decreasing, during the portion of and Cylmder Includes at least two preloaded Springs the stroke when the variable volume is increasing, or acting as a resonant spring-mass system and means for during the entire reciprocation cycle energizing the coil selectively during a portion of the cycle when the coil is moving through the magnetic Compressor accorclmg f 16 wherem fie|d there 18 included elastomeric cushioning members to 18. A compressor according to claim 17 wherein the Cushion the moving parts of the Variable Volume Chamcoil is energized only during that portion of the cycle at their P ints f ma imum excursion. when it is entering the magnetic field. 

1. A compressor for pressurizing a fluid comprising in combination: an elongate generally cylindrical closed housing having a major and minor axis and a first and second end defining first and second chambers in said housing; a first piston and cylinder disposed in the first chamber and mounted for relative reciprocation thereby defining a first variable volume fluid chamber; a second piston and cylinder disposed in the second chamber and mounted for relative reciprocation thereby defining a second variable volume fluid chamber; the first and second piston and cylinder combination mounted relative to each other for reciprocation along the major axis of the housing; means associated with each independent piston and cylinder combination to cause reciprocating motion of each piston and cylinder independent of the other; and means for admitting and removing a fluid to the first and second variable volume chambers.
 2. A compressor according to claim 1 wherein there is included means for limiting the excursion between each piston and cylinder.
 3. A compressor according to claim 1 wherein there is included means for cushioning the impact of the movable piston or cylinder as they reach the points of maximum excursion.
 4. A compressor according to claim 1 wherein the first and second pistons are fixed and the first and second cylinders are reciprocated in relation thereto.
 5. A compressor according to claim 1 wherein the first and second cylinders are fixed to the housing and the first and second pistons are reciprocated in relation thereto.
 6. A compressor according to claim 1 wherein the means for causing reciprocating motion between the piston and cylinder includes a linear motor having a coil mounted for movement into and out of a magnetic field.
 7. A compressor according to claim 3 wherein the impact cushioning means includes means for pressurizing the compressor housing with a working fluid at a pressure intermediate that of the maximum and minimum pressure in the variable volume during operation.
 8. A compressor according to claim 7 wherein the impact cushioning means further includes limit springs and elastomeric bumper pads.
 9. A compressor according to claim 6 wherein there is included means for limiting the excursion between each piston and cylinder said means including at least two preloaded springs acting as a resonant spring-mass system and the coil is energized only during the portion of the cycle when the coil is entering the magnetic field.
 10. A compressor according to claim 6 wherein the coil is mounted for movement so that it can be selectively energized during the portion of the stroke when the variable volume is decreasing, during the portion oF the stroke when the variable volume is increasing, or during the entire reciprocation cycle.
 11. A compressor for pressurizing a fluid comprising in combination: an elongate generally cylindrical closed housing having a major and minor axis and first and second ends defining first and second chambers in said housing; a first piston and cylinder disposed in the first chamber and mounted for reciprocation of the piston and the cylinder relative to each other thereby defining a first variable volume fluid chamber; a second piston and cylinder disposed in the second chamber and mounted for reciprocation of the piston and the cylinder relative to each other thereby defining a second variable volume fluid chamber; the first and second piston and cylinder combination mounted relative to each other for reciprocation along the major axis of the housing; means associated with each independent piston and cylinder combination to cause reciprocating motion of each piston and cylinder independent of the other; means for admitting a fluid to the first and second variable volume chambers; means for removing compressed fluid from the first and second variable volume chambers; means for controlling the emission and removal of fluid from each of the first and second variable volume chambers; and means for cushioning impact of the movable piston and cylinder as they reach the points of maximum excursion, said means includes pressurizing the compressor housing with a working fluid at a pressure intermediate that of the maximum and minimum pressure in the variable volume chambers during operation of the compressor.
 12. A compressor according to claim 11 wherein the impact cushioning means further includes limit springs and elastomeric bumper pads mounted for co-operation with the gas cushioning.
 13. A compressor according to claim 11 wherein the means for causing the reciprocating motion between the piston and cylinder includes a linear motor having a coil mounted for movement into and out of a magnetic field.
 14. A compressor according to claim 13 wherein there is included means for limiting the excursion between each piston and the cylinder said means including at least two preloaded springs acting as a resonant spring mass system and the coil is energized only during the portion of the cycle when the coil is entering the magnetic field.
 15. A compressor according to claim 13 wherein the coil is mounted for movement so that it can be selectively energized during the portion of the stroke when the variable volume is decreasing, during the portion of the stroke when the variable volume is increasing or during the entire reciprocation cycle.
 16. A compressor for pressurizing a fluid comprising in combination: an elongate generally cylindrical closed housing having a major and minor axis in a first and second end defining first and second chambers in said housing; a first piston and cylinder disposed in the first chamber and mounted for relative reciprocation thereby defining a first variable volume fluid chamber; a second piston and cylinder disposed in the second chamber and mounted for relative reciprocation thereby defining a second variable volume fluid chamber; the first and second piston and cylinder combination mounted relative to each other for reciprocation along the major axis of the housing; means associated with each independent piston and cylinder combination to cause reciprocating motion of each piston and cylinder independent of the other, said means including a linear motor having a coil mounted for movement into and out of a fixed magnetic field; means for admitting a fluid to the first and second variable volume chambers; means for removing compressed fluid from the first and second variable volume chambers; means for controlling the admission and removal of fluid from each of the first and second variable volume chambers; and means for limiting the excursion between the Piston and cylinder.
 17. A compressor according to claim 16 wherein said means for limiting the excursion between each piston and cylinder includes at least two preloaded springs acting as a resonant spring-mass system and means for energizing the coil selectively during a portion of the cycle when the coil is moving through the magnetic field.
 18. A compressor according to claim 17 wherein the coil is energized only during that portion of the cycle when it is entering the magnetic field.
 19. A compressor according to claim 16 wherein the coil is mounted for movement so that it can be selectively energized during the portion of the stroke where the variable volume is decreasing, during the portion of the stroke when the variable volume is increasing, or during the entire reciprocation cycle.
 20. A compressor according to claim 16 wherein there is included elastomeric cushioning members to cushion the moving parts of the variable volume chamber at their points of maximum excursion. 